1. Field of the Invention
The present invention relates to gradiometers, and more particularly to a free nuclear precession gradiometer for measuring the gradient of the total magnetic field of the earth by continuously analyzing the shape or, equivalently, the transverse time constant of the decaying precession signal.
2. Description of the Prior Art
Proton precession magnetometers have been in use since 1956. However, since the operarion of the instrument is discontinuous, the rotation of the instrument creates noise as do the ambient magnetic gradients. Thus the use of this instrument for high sensitivity applications generally has been superceded by the use of optically pumped helium or cesium magnetometers. Nevertheless, the line width of nuclear magnetometers is much less than the line width of optically pumped magnetometers. Therefore the potential sensitivity of the nuclear magnetometers is much greater if the rotation, continuity and orientation problems can be solved.
Some experimental uses of the proton nuclear magnetometer precession decay envelope to measure magnetic total field gradients in order to practice adjusting the degaussing coils of a ship were carried out in 1960. However, the sensitivities were orders of magnitude less than the 0.00003 gamma (.eta.T) per centimeter required for geophysical and marine surveys. The time constants for water are on the order of 2.8 seconds, requiring polarization every few seconds. Therefore, what is desired is a free nuclear precession gradiometer having a long decay time or virtually continuous operation with high sensitivity.